ARGUMENTS for and against same-sex marriage have very little impact on whether voters support it or not, a major report has found.

The finding presents a challenge to the rationale behind having a $122 million nationwide debate and postal survey on the issue.

Respondents were randomly assigned either the “plain” version of the question, “Do you approve or not approve of same-sex couples being allowed to marry?” or a version summarising the “equality” and ”religious freedom” arguments for and against.

But the different wording had “no discernible impact on attitudes”, the United States Studies Centre concluded, with “both arguments seem[ing] to carry equal force, cancelling each other out.”

Researchers also found Australians were far more accepting of same-sex marriage than their American counterparts, with 60 per cent of Aussies supporting it compared to just 48 per cent of people in the US.

That is despite same-sex marriage having been legal in some states for many years, and across the country since 2015.

In Australia, 32 per cent are opposed and 8 per cent undecided, while in the US, 40 per cent are opposed and 12 per cent are undecided.

The issue is also much more polarising between the two sides of politics in the US, with 70 per cent of Democrats approving of same-sex marriage to just 31 per cent of Republicans.

In Australia, 68 per cent of Labor supporters approve to 51 per cent of Coalition supporters, a much smaller spread.

Coalition voters are almost evenly split on the topic, while Labor voters are mostly in favour — suggesting it is an “easier” issue for the ALP, according to the report.

While partisan affiliation matters more in America and religion is significant in both, age is more important in Australia. Slightly less than half of voters over 55 supportive of same-sex marriage compared with approximately 73 per cent of those between 18 and 34.

In Australia, there is more difference in support for same-sex marriage between the age groups.Source:Supplied

In the US, 37 per cent of over 55s support same-sex marriage, compared with 59 per cent of 18- 34-year-olds.

There is also more religious opposition from people in the US. Just 36 per cent of American Christians are in favour of same-sex marriage, to 48 per cent of Australian Christians.

Same-sex marriage remains a contentious issue in the US even two years after the Supreme

Court’s ruling that state-level bans were unconstitutional. The USSC suggested “the reliance on the courts to provide some measure of legal clarity” may be part of the problem.

The report suggested religious freedom could be the next fight.

Americans are more likely to support the right of business to refuse to provide services to a same-sex marriage ceremony, with 50 per cent of Americans agreeing to this statement, to just 43 per cent of Australians.

The issue is currently being debated in the US, with a case before the Supreme Court over the right of businesses to refuse to provide services to a same-sex marriage ceremony.

Amendments for these rights of exemption have also been proposed to legislation in Australia.

In both countries, support for religious exemptions was highest among Christians. But in the

US, there was also a strong age difference, with young voters much less likely to support than those over 55, while in Australia there was little difference between younger and older voters.

The USSC at the University of Sydney surveyed more than 1000 people from Australia and more than 1000 from the United States.